The use of low density polyethylene in extrusion coating onto substrates such as paper and metal foils has historically been favored over that of polypropylene. This is a result of the relative poor extrusion coating characteristics of conventional polypropylenes at high throughput rates, where extension of the polymer melt through the extruder die increases. Conventional polypropylenes don't well-tolerate the higher extension, which adversely affects the orientation of the polymer melt, as evidenced by neck-in, draw resonance, edge weave and poor film quality.
Stabilizing additives are typically added to propylene polymer compositions to protect against degradation due to oxidation involving heat, UV radiation, ionizing radiation and transition metal impurities. In particular, in extrusion coating, fouling of the die or nip roll due to degradation products can occur if an appropriate level of stabilization is not present, potentially resulting in a shutdown of the extrusion coating product line. This is particularly important at higher extrusion temperatures. In addition to unit downtime, degradation can result in color development, undesirable taste or odor in the resulting polypropylene. Finally, appropriate stabilization levels are important to prevent large melt flow shifts at the operating temperature and residence time of the extruder, and to reduce the sensitivity of the extrusion coating operation to changes in operating conditions, e.g., extruder temperature, screw rpm, backpressure at the die, etc. Additives to inhibit degradation include free radical traps, the so-called primary antioxidants, and peroxide decomposers, sometimes referred to as secondary antioxidants. Hindered phenols and hindered amines are typical free-radical traps. Phosphites and thioesters are examples of peroxide decomposers. Phosphites are effective in the melt phase, and are used to prevent color generation. Thioesters are used for thermal stabilization to control undesirable taste and odor development in the polypropylene. They are effective in the solid phase.
Increasing the melt strength of the polypropylene is known to improve melt orientation. Techniques to improve melt strength in polypropylene have included irradiation of conventional flake polypropylene in reduced-oxygen environments, as described, for example, in U.S. Pat. Nos. 4,916,198, 5,047,485, 5,414,027, 5,541,236, 5,554,668, 5,591,785, 5,731,362, and 5,804,304. For example, U.S. Pat. No. 5,508,318 discloses compounded blends of irradiated and non-irradiated olefin polymer materials suitable for extrusion coating applications requiring low gloss. These irradiation methods increase propylene polymer melt strength by creating polymer radicals during irradiation which then re-combine to form long-chain branches in the reduced oxygen environment. Conventionally, phenolic antioxidants have long been used to improve polymer stability under elevated temperature conditions, such as those typically experienced during extrusion, or during extended periods of storage. However, their use in irradiated compositions undermines enhanced melt strength by scavenging free radicals, thereby reducing the number of polymeric free radicals available to recombine to form long-chain branches. Moreover, irradiation of phenolic antioxidant-containing polymers can result in the formation of degradation products that impart undesirable color. Non-phenolic stabilizers have been used in the irradiation of conventional polyolefin materials to avoid such problems, as described in U.S. Pat. No. 6,664,317. International Publication No. WO 2009/003930 discloses irradiation of high melt strength polypropylene in the form of pellets containing non-phenolic antioxidants. However, a continuing need exists for extrusion coating processes that provide good film quality at high line speeds.